1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to an organic electroluminescent device and a fabricating method thereof.
2. Discussion of the Related Art
In general, an organic electroluminescent device (ELD) emits light by injecting electrons from a cathode and holes from an anode into an emission layer, combining the electrons with the holes, generating an exciton, and transitioning the exciton from an excited state to a ground state. Contrary to a liquid crystal display (LCD) device, an additional light source is not necessary for the organic ELD to emit light because the transition of the exciton between states causes light to be emitted. Accordingly, the size and weight of the organic ELD can be reduced. The organic ELD has other excellent characteristics such as low power consumption, superior brightness, and fast response time. Because of these characteristics, the organic ELD is regarded as a promising candidate for next-generation consumer electronic applications, such as cellular phones, car navigation systems (CNS), personal digital assistants (PDA), camcorders, and palmtop computers. Moreover, since fabricating the organic ELD is a simple process with few processing steps, it is much cheaper to produce an organic ELD than an LCD device.
Two different types of organic ELDs exist: passive matrix and active matrix. While both the passive matrix organic ELD and the active matrix organic ELD have a simple structure and are formed by a simple fabrication process, the passive matrix organic ELD requires a relatively high amount of power to operate. In addition, the display size of a passive matrix organic ELD is limited by its structure. Furthermore, as the number of conductive lines increases, the aperture ratio of a passive matrix organic ELD decreases. In contrast, active matrix organic ELDs are highly efficient and can produce a high-quality image for a larger display with relatively low power.
FIG. 1 is a cross-sectional view of an organic ELD according to the related art. In FIG. 1, an array element 14 including a thin film transistor (TFT) “T” is formed on a first substrate 12. A first electrode 16, an organic electroluminescent layer 18, and a second electrode 20 are formed over the array element 14. The organic electroluminescent layer 18 may separately display red, green, and blue colors for each pixel region. Generally, separate organic materials are used to emit light of each color for the organic electroluminescent layer 18 in each pixel region. An organic ELD is encapsulated by attaching the first substrate 12 and a second substrate 28, which includes a moisture absorbent material 22, with a sealant 26. The moisture absorbent material 22 eliminates moisture and oxygen that may penetrate into a capsule of the organic electroluminescent layer 18. After etching a portion of the second substrate 28, the etched portion is filled with the moisture absorbent material 22, and the filled moisture absorbent material is fixed by a holding element 25.
FIG. 2 is a plan view of an organic ELD according to the related art. In FIG. 2, a switching element TS, a driving element TD, and a storage capacitor CST are formed in each pixel region on a substrate 12. The switching element TS and the driving element TD can be a combination of at least two thin film transistors (TFTs) according to the operating requirements of the organic ELD. The substrate 12 is made of a transparent insulating material, such as glass or plastic. Moreover, a gate line 32 and a data line 34 cross each other with an insulating layer (not shown) in between the gate line 32 and the data line 34. A power line 35 is placed in parallel to and separated from the data line 34. Two TFTs are used as the switching element TS and the driving element TD. The switching element TS includes a gate electrode 36, an active layer 40, a source electrode 46, and a drain electrode 50. The driving element TD includes a gate electrode 38, an active layer 42, a source electrode 48, and a drain electrode 52. The gate electrode 36 and the source electrode 46 of the switching element TS are connected to the gate line 32 and the data line 34, respectively. The drain electrode 50 of the switching element TS is connected to the gate electrode 38 of the driving element TD through a first contact 54. The source electrode 48 of the driving element TD is connected to the power line 35 through a second contact 56. The drain electrode 52 of the driving element TD contacts a first electrode 16 in a pixel region P. The power line 35 overlaps the first electrode 16, which is composed of polycrystalline silicon, with an insulating layer interposed between the power line 35 and the first electrode 16 to form a storage capacitor CST.
FIG. 3 is a cross-sectional view of the organic ELD shown of FIG. 2 taken along III-III according to the related art. In FIG. 3, a driving element TD including a gate electrode 38, an active layer 42, a source electrode 56, and a drain electrode 52 is formed on a substrate 12. A first electrode 16 contacting the drain electrode 52 of the driving element TD with an insulating layer interposed between the first electrode 16 and the drain electrode 52 is formed over the driving element TD. An organic electroluminescent layer 18 emitting light of one color is formed on the first electrode 16, and a second electrode 20 is formed on the organic electroluminescent layer 18. The organic electroluminescent layer 18, the first electrode 16, and the second electrode 20 constitute an organic electroluminescent diode DEL. A storage capacitor CST including first capacitor electrode 15 and second capacitor electrode 35 and the driving element TD are electrically connected in parallel to the switching element TS (as shown in FIG. 2). The second capacitor electrode 35 is connected to a power line. The source electrode 56 of the driving element TD is connected to the second capacitor electrode 35. The second electrode 20 covers the driving element TD, the storage capacitor CST, and the organic electroluminescent layer 18.
FIG. 4 is an equivalent circuit diagram of an organic ELD according to the related art. In FIG. 4, a data line 34 is in parallel to and separated from a power line 35. A gate line 32 crosses the data line 34 and the power line 35 to define a pixel region P. A switching element TS, a driving element TD, and a storage capacitor CST are disposed in the pixel region.
In the organic electroluminescent device according to the related art, the power line 35 limits the area of the organic electroluminescent layer. As the area of the electroluminescent layer decreases, the current density required to obtain the same brightness increases. Increasing the current density shortens the expected life span of an organic ELD. An increased current density is required to obtain sufficient brightness in a bottom emission organic ELD because the use of at least three lines causes a reduction in the aperture ratio. Moreover, as the number of conductive lines increases, the probability of defects in the conductive lines increases resulting in a decrease in the production yield.